"This is why discovering a galaxy cluster at a redshift of 2.07 feels like unearthing a rare and valuable gem," says Gobat. Redshift is a measure of the change in frequency in light waves coming from an object that is moving away from the observer.

"As well as being at a record-breaking distance, what makes this unique is that it's not a proto-cluster undergoing formation, but an already mature, proper galaxy cluster."

Accidental find

According to the researchers, the chance discovery of CL J1449+0856 occurred during analysis of infrared data from NASA's Spitzer Space Telescope.

"The structure caught our eyes as a clump of galaxies which appeared much redder and more concertrated than those in the surroundings," says co-author Emmanuele Daddi, also of CEA.

Using the Very Large Telescope in Chile the astronomers determined how far away the cluster was. Hubble Space Telescope images showed most of the galaxies in the cluster won't forming new stars and were dominated by cool red stars more than a billion years old.

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